A birefringent material can be used for many purposes. One example is to separate light, such as light rays, images, or the like, and/or to change the direction or location of such light. Such separation and such direction or location changing can be used for a variety of purposes. Examples of devices using birefringent materials are disclosed in U.S. Pat. Nos. 5,572,341 and 5,715,029 and commonly owned U.S. application Ser. No. 08/817,846, filed Apr. 25, 1997, the entire disclosures of which hereby are incorporated by reference. In several embodiments of devices disclosed in such patent applications either the light is transmitted directly through a birefringent material without a change in direction or the direction or location of the light is shifted.
An example of a conventional birefringent material is a crystal made of calcite (calcium carbonate) CaCO.sub.3. In the above-mentioned exemplary devices calcite or other birefringent material is able to transmit light directly through the device or to shift or to translate the location of the light, for example, depending on the direction of the electric vector of plane (linear) polarized light relative to the axis of the calcite.
The calcite crystal used in such exemplary devices is birefringent and it is cut or so arranged that it has coplanar surfaces on which light is incident and from which light exits. The direction of incident and exiting light relative to the coplanar surfaces may be normal (perpendicular) to the respective planes of those surfaces. For exemplary descriptive purposes, one axis of the calcite is horizontal and the other is vertical, and the vertical axis either tips forward or backward such that it does not lie in a plane that is parallel with the mentioned coplanar surfaces. For incident plane or linearly polarized light which is incident on one of the coplanar surfaces, if the plane of polarization is perpendicular to the mentioned tilted axis of the calcite crystal, then the light is transmitted directly through the calcite crystal without a change in direction or location of the light; but if the plane of polarization is aligned with the mentioned axis, then there is a shift in the direction of the light and the location where the light exits the calcite crystal is different from the location that the first-mentioned light exits the calcite crystal. The displacement between the two light beams or light rays exiting the calcite crystal depends on the birefringence of the crystal and the thickness of the crystal.
Calcite crystal material of optical quality is relatively expensive. Also, usually the area or size of calcite crystal material is relatively small.
It would be desirable to obtain or to make birefringent material, especially that of good optical quality, that can be used in visual display and viewing devices, for example, while reducing the cost of that material relative to the cost of calcite crystal material. It also would be desirable to facilitate making such material and to enable the making of the material in a variety of sizes, both in terms of area on which light may be incident and thickness.
Birefringent material has different indices of refraction characteristics for different directions or different directions of plane of polarization of light transmitted therethrough. Various film or sheet materials, especially those that are stretched, are known to have birefringence characteristics. For example, a stretched film of polyvinyl alcohol is known to be uniaxial, birefringent, and optically positive. The optical axis usually is in the direction of stretch. Polystyrene is another example of a uniaxial, birefringent material, but polystyrene usually is referred to as being optically negative. The uniaxial films usually have two indices of refraction, one which is in the direction of stretch and the other which is generally perpendicular to the stretched direction.
A polyester film material, such as that sold under the trademark Mylar, is a biaxial material. The biaxial material typically has three indices of refraction. Looking at a top plan view of the material, one axis or index of refraction would be in the direction of stretch or linear extent of the film material and generally in the plane of the material; a second would be perpendicular to the first and also in the generally in the plane of the material; and a third index of refraction would be looking through the material at an edge view of it. In a biaxial film material, such as Mylar, there are two axes along which there is no birefringence exhibited by the material. Those axes typically are referred to as the optical axes or optic axes.